The invention relates to a window panel and more particularly to a high efficiency light transmitting window panel for passing incident solar light into a room while simultaneously blocking the escape of heat energy by radiation, convection or conduction from the room.
In the design of buildings, it is desirable that the windows efficiently pass solar radiation while blocking the escape of heat from the inside of the building by either radiation, conduction or convection. Furthermore, it is desirable that such solar radiation be transmitted through the window over as wide a range of incident angles as possible.
In some prior art windows one or more panes of transparent glass or plastic are separated by air. Such double pane windows are somewhat effective in blocking the escape of heat by means of conduction since the air chamber between the panes acts as a barrier to direct conductive heat loss. However, convective air currents within the air chamber are generated by the temperature differential between the panes and this causes a significant heat loss. Also heat generated or reemitted (in the form of infrared light, for example) from within the building passes easily through the window, causing further heat loss. Still another problem of such double pane windows is that much of the light is lost which strikes the window at large incident angles (measured with respect to the normal to the outer pane.) Thus, the closer the incident light comes to being tangential to the window, the less efficient the window becomes in transmitting the incident light.
The light transmitting characteristics of the ideal window should be independent of the angle of incidence or polarization, thus making the window useful even on a cloudy day. The window should also be made of low cost materials without sacrificing sturdiness.
Some of these requirements are contradictory. For example, in order to make the window sturdy or in order to make it more effective in blocking the escape of heat energy, some lower light transmitting qualities than relatively thinner windows. Thus, the light transmitting capability is degraded to improve the capability to block the escape of heat energy. Furthermore, when the window is made relatively thick the light acceptance angle properties may be degraded in some prior art arrangements.